Leaves of indigo plants (indigo-plant leaves) contain indican (from which indigo is made). Therefore, the leaves have been preferentially used as a starting material for producing a blue dye around the world. Nowadays chemically synthesized indigo dyes are often used in industry; however, indigo-plant leaves continue to be grown for use as a dye.
In particular, leaves of Polygonum tinctorium, which is an indigo plant predominantly grown in Japan, are known to contain various useful ingredients in addition to indican. For example, the leaves of this indigo plant are known to contain a large amount of tryptanthrin (structural formula shown below).

It has been reported that tryptanthrin has an antibacterial action against fungi of the genus Malassezia and is therefore effective against atopic dermatitis in which fungi of the genus Malassezia are involved (Patent Literature (PLT) 1). Tryptanthrin has been reported to have an effect of suppressing type IV allergic reactions (Patent Literature (PTL) 2).
It is thus becoming increasingly evident that tryptanthrin exhibits useful effects particularly when applied to the skin, and demand for tryptanthrin is increasing.
As a method for extracting tryptanthrin from a indigo plant, extraction from a indigo plant by using an organic solvent is known. For example, an extraction method using dichloromethane as an extractant (Patent Literature (PTL) 1) and a method using 1,3-butylene glycol or the like as an extractant are known. (Very little tryptanthrin can be extracted with water.)
However, the use of an organic solvent is likely to result in extraction of a highly irritating ingredient. Therefore, depending on the type of organic solvent used, the obtained extract may be unsuitable for application to the skin or may contain tryptanthrin in an unstable state. No extraction method that is suitable for application to the skin has been developed yet. Another problem is that tryptanthrin fundamentally has poor light stability.